At present, prepumping to a rough vacuum (or backing vacuum as it is often called) is usually carried out with an oil-sealed rotary pump which is both lubricated and sealed with hydrocarbon or fluorocarbon oil. Some of the oil molecules are degraded and fragmented into smaller molecules during the operation of the rotary pump and these small hydrocarbon and fluorocarbon molecules exhibit a high vapour pressure relative to that of the oil before the latter was used in the pump. It is difficult to prevent these small molecules from passing back from the pump and entering the vacuum vessel where they contaminate all the surfaces of the vessel and its contents by coating them with an adherent oily film.
In the more modern electron microscopes, in which the specimen is not heated by the electron beam to the extent that it was in earlier types of electron microscopes, oil vapour arising from the oil lubricated pumping system condenses to form a contamination on the specimen, obscuring fine detail and reducing resolution by acquiring an electrical charge which deflects the electron beam. Furthermore, in the scanning transmission type of electron microscope which is now in use, the electron beam is produced from a tungsten tip by field emission, and the presence of any oil vapour in the surrounding vacuum will seriously affect the stability of the electron beam current.
In the technology of producing integrated silicon chip circuits in a high vacuum environment, the presence of any oil vapour is likely to render the chip inoperative because of the deposition of a thin oil film which may prevent good contact between layers and may insulate segments which are intended to be electrically connected.
In these fields, and others, oil lubricated pumps have continued to be used but elaborate systems have been developed for condensing out oil vapour or otherwise preventing it from reaching critical sites. One such system utilises a trap filled with pellets of alumina or zeolite, or a trap maintained at liquid nitrogen temperature, in the pumping line connecting the backing pump with the high vacuum pump. However, these traps are never completely effective in condensing out the oil vapour, so some contamination of the vessel with oily vapour always occurs.
At present, the only oil-free pumps capable of prepumping a vessel down from atmospheric pressure to fractions of a mm Hg are sorption pumps but the use of these is time-consuming and expensive. Sorption pumps usually consist of a stainless steel canister filled with zeolite pellets which, when cooled to liquid nitrogen temperature, have the ability to absorb most atmospheric gases. The canister is first heated and pumped with a backing pump (which needs to be fitted with an oil trap) to remove air from the zeolite pellets. It is then removed from the backing pump, connected to the vessel to be evacuated and then cooled to liquid nitrogen temperature, whereupon it begins pumping and continues to do so until the zeolite becomes saturated with air. The pump must then be disconnected from the vacuum vessel and reprocessed by heating and pumping and again cooling with liquid nitrogen. Sorption pumps were invented to provide oil-free prepumping of systems which are to be evacuated to a very high vacuum by oil-free pumps such as sublimation pumps, ionization pumps or cryopumps. Despite the cost of the liquid nitrogen used for cooling them and the inconveniences involved in processing them, they are widely used for such purposes.
Those oil-free mechanical vacuum pumps which are commercially available are quite incapable of producing high vacuum. Two existing commercial pumps of this type employ split polytetrafluoroethylene (PTFE) sealing rings backed by a split, spring-steel band. The claimed performance of these pumps against atmosphere is 23 mm Hg (absolute) in one case and 124 mm Hg (absolute) in the other, and a clear limiting factor on performance is the split in the steel band which would allow a degree of air leakage. As backing pumps, the utility of these pumps in the applications discussed above is limited to prepumping prior to the use of sorption pumps.
A further mechanical oil-free pump developed by the present applicant is disclosed in Australian Pat. No. 481072. This pump was found capable of producing high vacuum conditions without the use of lubricating and sealing oil but the vacuum which could be achieved was limited by difficulties in sealing against gas leakage into the working space of the pump and by the need to have valves which had to be subjected to gas pressure to open. The vacuum which could be produced in the high vacuum stage of a multi-stage pump was then determined by the pressure required to open an exhaust valve in the high vacuum stage of the pump.
Improvements which have proven successful in meeting these difficulties are disclosed in Australian Pat. No. 516210 and in co-pending Australian patent application No. 68083/81. According to Australian Pat. No. 516210 gas passes from a cylindrical working space above the piston to an annular working space below the piston by way of a gas transfer passage opening at the end face of the cylinder above the piston. Co-pending Australian Patent application No. 68083/81 discloses alternative sealing ring assemblies which have proven especially effective in enhancing the sealing of the cylinder.